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«KOALA–AAOmega Manual Volume I: User Guide Andy Green Version 1.0.2 — Dated: 28 April 2016 Please read How to use this manual on the inside of the ...»

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KOALA–AAOmega Manual

Volume I: User Guide

Andy Green

Version 1.0.

2 — Dated: 28 April 2016

Please read How to use this manual on the inside of the cover.

Parts of this manual:

Volume I: User Guide

Part I Overview of KOALA and AAOmega............... 4

Part II Preparing for observing................... 12

Part III Observing with KOALA+AAOmega.............. 24 Part IV Data Reduction...................... 54 Part V Supplementary Information (separate download)......... 68 Volume II: Support Manual Guide for AAO staff and Troubleshooting Instructions.

The Australian Astronomical Observatory www.aao.gov.au Compiled: Sun 1st May, 2016 How to use this manual This AAO Instrument Manual is designed to be a complete reference for the typical user.

It is divided into parts, with each part relevant for a particular phase of a program:

Part I Material relevant for preparing a proposal. An overview of the instrument, its capabilities and its overheads is provided.

Part II Material relevant for preparing for awarded time, including details on creating an observing plan, what information or observation description files must be prepared in advance, and other practicalities.

Part III How to operate the instrument and other tasks required at the telescope. Users should be familiar with this part in advance, but certainly need not memorise the whole thing.

Part IV Overview of reducing data.

Part V Supplementary information relevant only to a few observers. This section is often offered as a separate download on the website.

The division of the manual means it is not necessary to read and understand more than one part at any one time.

The manual has been designed with print and on-screen readers in mind, and has hyperlinks throughout to aid in quickly navigating the document.

The AAO welcomes and appreciates feedback on this document. Errors, mistakes, omissions, etc, cannot be corrected if we are not aware of them. Talk to your support astronomer.

Printed copies of these manuals are kept in the observing control rooms, and users are invited to mark changes or problems directly on those copies.

The KOALA–AAOmega Manual — Volume I: User Guide I Part Overview of KOALA and AAOmega

–  –  –

This part is a section of the larger KOALA–AAOmega manual, which is available from the website.

http://www.aao.gov.au/science/instruments/current/koala/overview Chapter 1 KOALA Overview KOALA, the Kilofibre Optical AAT Lenslet Array, is a wide-field, high efficiency, integraleld unit designed for use with the bench mounted AAOmega spectrograph on the AAT. The KOALA “front end” is described here, and the AAOmega spectrograph is described in the next chapter, AAOmega Overview.

KOALA has 1000 hexagonal lenslets arranged in a rectangular array. The field of view is selectable between either 15.3×28.3 arcsec with 0.7 spatial sampling or 27.4×50.6 arcsec with

1.25 sampling. To achieve this, KOALA uses a telecentric double-lenslet array fed by interchangeable fore-optics. The IFU is mounted at the f/8 Cassegrain focus and feeds AAOmega via a 31m fibre run.

KOALA benefits from all of the flexibility of the reconfigurable AAOmega spectrograph.

The double beam spectrograph provides user selectable wavelength coverage and resolution using a series of movable, interchangeable gratings. A set of low, medium, and high resolution gratings provide R ∼ 1, 000, R ∼ 5, 000 and R ∼ 10, 000 across the wavelength range 330 nm to 1000 nm. Full spectral coverage is possible in a single exposure with the low-resolution gratings.

˚ The overall efficiency of KOALA, AAOmega and the AAT is about 4% at 3700 A and 18% ˚ at 6563 A in the standard setup.

–  –  –

Figure 1.2: The KOALA head unit installed in the Cassegrain cage of the AAT.

1.1. MICRO-LENS ARRAY Figure 1.3: The KOALA micro lens arrays. The left image shows the SUSS hexagonal lenslets which face the sk, with a 100-micron grid overlaid. The right image shows the second, circular lenslet array which faces the fibres.

1.1 Micro-lens Array KOALA uses a double microlens array to feed the fibres (shown in Figure 1.3. In principle it is possible to feed a fibre array directly by placing it at the (magnified) focal plane, but a microlens array offers two significant advantages. First, a microlens array has close to 100% filling-factor, whereas a fibre array has a limited filling factor due to the fibre cladding and packing. Secondly, a microlens array can be used to form a pupil-image on the face of the fibre, and therefore the coupling into the fibre is more efficient because both the pupil image size and the focal ratio can be matched to those required for efficient coupling into the fibre.

Two options were considered for the KOALA: a single microlens and a double microlens.

A single microlens is the simplest system and is the system that has been traditionally used to feed fibre IFUs. The double microlens system is novel and offers the advantage that the fibres feed is always telecentric, thus minimising losses.

1.2 Focal plane and field of view KOALA offers a rectangular field of view with a selectable scale. The 1000 lenslets of the focal plane are laid out in a rectangle of 40 × 25 lenslets, or an aspect ratio of 0.54 (see Figure 1.4).

The field of view is selectable between 15.3 × 28.3 arcsec and 27.4 × 50.6 arcsec, with spatial sampling of 0.7 arcsec and 1.25 arcsec, respectively. The field is oriented such that at position angle of 0◦, the long axis is east-west. The position angle is measured north-through east. For mechanical reasons, the position angle is restricted to the range 0◦ to 180◦ (additional range is redundant).

Each lenslet covers a hexagonal patch on the sky, and the lenslets contiguously cover the field of view. The fill factor of the lenslets is 96%. The sampling of either 0.7 arcsec or 1.25 arcsec is centre-to-centre.


–  –  –

Figure 1.4: Layout of KOALA Lenslets.

The top shows the FoV in the wide field of view configuration (1.25 sampling), and the bottom in the narrow FoV configuration (0.7 sampling). Dead fibres/lenslets (as known on 17 October 2013) are shown in black.

Chapter 2 AAOmega Overview

–  –  –

AAOmega is a dual-beam spectrograph. Science fibres are arranged into a pseudo-slit which feeds into a single collimator and then separates into the blue and red arms of the system via a dichroic beam splitter. There are two dichroics, one operates at 570nm and one at 670nm. Each arm of AAOmega uses one of a selection of Volume Phase Holographic (VPH) gratings. The system shutter is in front of the fibre pseudo-slit, so both cameras must use the same exposure time.

AAOmega can be configured to observe the entire optical spectrum over the wavelength range 370nm-900nm, with a small overlap between the red and blue arms around the dichroic wavelength (570 or 670nm). The grating set available allows a range of resolutions between R ∼ 1, 000 and R ∼ 10, 000. The fibre spectra are recorded onto the 2K×4K E2V CCDs with light dispersed along the 2K axis (not the 4K axis). Hence, at low resolution the entire accessible spectral range is recorded at once, but at higher resolutions the user must tune the wavelength range to that which best suits their requirements.

AAOmega uses Volume-Phase Holographic (VPH) transmission gratings. These have flexible blaze angles. Each grating has a specific design blaze angle which will give the absolute


maximum efficiency with that grating (the super blaze). This peak efficiency reduces smoothly with wavelength away from that. The usual setup for most programs is therefore to have the grating set at its super blaze angle and the camera at twice this angle to centre the maximum efficiency wavelength on the CCD. The complication comes when the observer wishes to observe at a central wavelength which is some distance away from the super blaze angle for the grating. This would mean observing with the grating and camera angles highly asymmetric, and therefore operating on the low efficiency (and rapidly falling) part of the blaze envelope for the grating. The solution is to tune the grating and camera angles to new values. This shift in the grating angle will shift the blaze profile away from the super blaze, flattening the steep wings of the super blaze envelope and boosting system performance at the desired wavelength(s), with the expense of a slight reduction in overall peak performance in comparison to the super blaze setting.

2.1 References • “AAOmega: a scientific and optical overview” : Saunders et al. 2004 SPIE 5492 389 • “AAOmega: a multipurpose fiber-fed spectrograph for the AAT” : Smith et al. 2004 SPIE • “Performance of AAOmega: the AAT multi-purpose fiber-fed spectrograph” : Sharp et al. 2006 SPIE 6269E 14, arXiv:astro-ph/0606137 • “Optimal Extraction of Fibre Optic Spectroscopy” : Sharp & Birchall 2010 PASA 27(1) 91, arXiv:0912.0558 • “Sky subtraction at the Poisson limit with fibre-optic multi-object spectroscopy” : Sharp & Parkinson MNRAS, 2010, 408, 2495 The KOALA–AAOmega Manual — Volume I: User Guide

–  –  –

This part is a section of the larger KOALA–AAOmega manual, which is available from the website.

http://www.aao.gov.au/science/instruments/current/koala/overview Chapter 3 In advance of your observing run

1. Contact your support astronomer (see the AAT Schedule). Make sure you discuss with


• What your program is and your observing strategy, including exposure times;

• Recent performance of the instrument (e.g., how fast will field reconfiguration times be for 2dF);

• Any questions you have about observation description files, which must be prepared in advance (e.g.,.FLD files for 2dF, finder charts for KOALA, observing scripts, etc.);

• Which particular mode/setup you plan to use for your program.

• When you will be arriving at the telescope or remote observing site.

2. Fill out your Travel Form, regardless of whether you will be observing remotely or at the AAT.

This allows the AAO to make appropriate reservations, etc.

3. Read this documentation, especially Parts II: Preparing for observing and III: Observing with KOALA+AAOmega. Users of 2dF must be prepared to use configure at the telescope.

4. You should plan to arrive early, preferably the day before your first night on the telescope, especially if this will be your first observing run with this particular instrument/telescope. This will give you time to discuss your program with your support astronomer in detail, familiarise yourself with the data reduction software, and the computing and observing system at the telescope or remote observing site.

5. If observing with 2dF, prepare your.FLD configuration files. If observing with another instrument, prepare finding charts for your targets. Preparing.FLD files is a complex task, and should not be left until the last minute.

Astronomers are strongly encouraged to reduce their data in real time at the telescope.

Although such “quick-look” reductions often require revisiting afterwards, they are crucial to ensuring the best quality data is obtained. AAOmega and HERMES data are reduced using the 2dfdr software environment. Reduction facilities are available at the AAT and via the remote observing system, but users may wish to download and run the software e.g., on their laptop.

The 2dfdr webpage provides all necessary links and information for the data reduction task.


Chapter 4 Preparing to Observe with KOALA

4.1 Finding Charts and Target Acquisition Positive target acquisition with KOALA is best accomplished with finding charts. Good finding charts should

• show an area of the sky about 3–5 arcminutes on a side,

• resemble the V -band image,

• show North and East,

• be centred on the acquisition star, not the final target, unless no acquisition star is used,

• should include a separate image of the final science target1 (if it is not visible on the finder image above),

• include offsets from the acquisition star to the target in arcseconds on the sky,

• include the coordinates (in HH MM SS.SS, DD MM SS.SS format) of both target and acquisition star,

• print well in black and white.

KOALA on the AAT can accurately acquire an unresolved, 18 mag target in about 5 minutes. The accuracy/repeatability of the acquisition is better than 1.0 arcsec night to night, and less than a few arcseconds run to run. Fainter and/or unresolved targets will be less accurately acquired.

For more accurate acquisitions, and for faint or diffuse targets, an offset star must first be acquired. A nearby (r 200 arcsec) bright (10 mV 14) star is acquired and imaged with KOALA. With the telescope guiding, a guided offset is performed to the science target. A world-coordinate system can then be created a priori for the final data using the star in the acquisition frame as a reference.

4.2 Dithering on target and offset skies It is recommended to dither the telescope a small amount between each exposure in order to reduce the impact of defects in the instrument on your data. Dithering will help fill in dead lenslets/fibres in the array, and reduce the imprint of the instrument characteristics on the final data.

1 It is often helpful to include an overlay of the KOALA FoV centred on the target, and also showing any offset sky positions to be used.


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